Sidewall-free CESL for enlarging ILD gap-fill window

ABSTRACT

An integrated circuit structure includes a first gate strip; a gate spacer on a sidewall of the first gate strip; and a contact etch stop layer (CESL) having a bottom portion lower than a top surface of the gate spacer, wherein a portion of a sidewall of the gate spacer has no CESL formed thereon.

PRIORITY CLAIM AND CROSS-REFERENCE

This application is related to copending U.S. patent application Ser. No. 13/912,449, entitled “Sidewall-Free CESL for Enlarging ILD Gap-Fill Window,” filed on Jun. 7, 2013, with this application being a continuation of, and the copending '449 application being a divisional of, U.S. application Ser. No. 12/750,485, entitled “Sidewall-Free CESL for Enlarging ILD Gap-Fill Window,” filed on Mar. 30, 2010, and U.S. Provisional Application No. 61/186,954 filed on Jun. 15, 2009, entitled “Sidewall-Free CESL for Enlarging ILD Gap-Fill Window,” which applications are hereby incorporated herein by reference.

TECHNICAL FIELD

This invention relates generally to integrated circuits, and more particularly to the gap-filling of inter-layer dielectrics (ILDs) in the manufacturing of integrated circuits.

BACKGROUND

Replacement gates are widely used in the manufacturing of integrated circuits. In the formation of replacement gates, polysilicon gates are formed first, and replaced by metal gates in subsequent process steps. With the using of replacement gates, the gates of PMOS and NMOS devices can have band-edge work functions, so that their performance can be optimized.

The replacement gates typically have great heights, and hence the aspect ratios of the gaps between gate stacks are also high. For example, FIG. 1 illustrates gate polys 102 and 104 adjacent to each other. Gap 106 is thus formed between gate polys 102 and 104. After the formation of gate polys 102 and 104, contact etch stop layer (CESL) 108 may be formed. The formation of CESL 108 adversely results in an increase in the aspect ratio of gap 106.

Referring to FIG. 2, inter-layer dielectric (ILD) 110, often referred to as ILD0, is formed to fill gap 106. In subsequent process steps, gate polys 102 and 104 may be replaced with metal gates. Currently, high-density plasma (HDP) processes are widely used for the ILD0 gap filling process. However, the gap filling capability of HDP is not satisfactory, and hence void 112 may be formed in gap 106. If formed using advanced technologies such as 22 nm or 20 nm technologies, the aspect ratio of gap 106 is particularly high. What is needed, therefore, is a method and structure for overcoming the above-described shortcomings in the prior art.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present invention, and the advantages thereof, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which:

FIGS. 1 and 2 illustrate cross-sectional views of intermediate stages in a conventional manufacturing process of an integrated circuit structure; and

FIGS. 3A through 9 are cross-sectional views and top views of intermediate stages in the manufacturing of an integrated circuit structure in accordance with an embodiment.

DETAILED DESCRIPTION

The making and using of the embodiments of the present invention are discussed in detail below. It should be appreciated, however, that the embodiments provide many applicable inventive concepts that can be embodied in a wide variety of specific contexts. The specific embodiments discussed are merely illustrative of specific ways to make and use the invention, and do not limit the scope of the invention.

A novel integrated circuit structure and a method of forming the same are provided. The intermediate stages of manufacturing an embodiment are illustrated. The variations of the embodiment are then discussed. Throughout the various views and illustrative embodiments, like reference numbers are used to designate like elements.

FIG. 3A illustrates a cross-sectional view of an integrated circuit structure. Substrate 10 is provided. Substrate 10 may be formed of commonly known semiconductor materials such as silicon, silicon germanium, gallium arsenide, and the like. First gate stack 21 and second gate stack 41 are formed on substrate 10. First gate stack 21 includes gate dielectric 20, gate strip 22, and optional hard mask layer 24. Gate spacers 26 are formed on sidewalls of gate stack 21. Second gate stack 41 includes gate dielectric 40, gate strip 42, and optional hard mask layer 44. Gate spacers 46 are formed on sidewalls of gate stack 41. Gate spacers 26 and 46 are adjacent to each other with gap 34 therebetween.

In an embodiment, gate strips 22 and 42 are formed of polysilicon. In other embodiments, gate strips 22 and 42 are formed of other conductive materials such as metals, metal silicides, metal nitrides, and the like. A common source or a common drain 30 (referred to as a source/drain hereinafter) may be located in substrate 10 and between gate stacks 21 and 41. Source/drain regions 36 and 48 may be formed adjacent to gate stacks 21 and 41, respectively. Further, silicide regions 32 may be formed on source/drain regions 30, 36, and 48. Gate stack 21 and source/drain regions 30 and 36 form a first MOS device, and gate stack 41 and source/drain regions 30 and 48 form a second MOS device.

FIG. 3B illustrates an alternative embodiment, wherein gate (poly) strips 22 and 42 are formed directly over shallow trench isolation (STI) region 50. Also, the structure shown in FIG. 3B may be the extension of the structure shown in FIG. 3A. A top view of the structure shown in FIGS. 3A and 3B is illustrated in FIG. 3C.

FIG. 4 illustrates the formation of contact etch stop layer (CESL) 52, which may be formed of commonly used CESL materials including, but not limited to, SiN_(x), SiO_(x), SiON, SiC, SiCN, BN, SiBN, SiCBN, and combinations thereof. In an embodiment, CESL 52 is formed using plasma enhanced chemical vapor deposition (PECVD), although other methods such as sub atmospheric chemical vapor deposition (SACVD), low pressure chemical vapor deposition (LPCVD), atomic layer deposition (ALD), high-density plasma (HDP), plasma enhanced atomic layer deposition (PEALD), molecular layer deposition (MLD), plasma impulse chemical vapor deposition (PICVD), and the like can also be used.

In an embodiment, CESL 52 includes top portions 52-1, sidewall portions 52-2, and bottom portions 52-3. Top portion 52-1 is located on the top of hard mask layers 24 and 44. Sidewall portions 52-2 are located on the sidewalls of gate spacers 26 and 46. The bottom portions 52-3 are at the bottom of gap 34 and on silicide regions 32. Sidewall portions 52-2 have different characteristics from top portions 52-1 and bottom portions 52-3. In an embodiment, sidewall portions 52-2 have a density lower than, for example, about 80% percent, of the densities of top portions 52-1 and bottom portions 52-3.

An exemplary formation process of CESL 52 is performed using PECVD. The PECVD for forming CESL 52 may include generating plasma using a low-frequency energy source that provides a low-frequency energy, wherein the frequency of the low-frequency energy may be lower than about 900 KHz. An exemplary low frequency is about 350 KHz. Further, for generating the plasma, a high-frequency energy source is also used to provide a high-frequency energy. The frequency of the high-frequency energy may be greater than about 900 KHz. An exemplary high frequency is 13.56 MHz. Throughout the description, the power provided through the low-frequency energy source is referred to as a low-frequency power, while the power provided through the high-frequency energy source is referred to as a high-frequency power. The high-frequency power and the low-frequency power may be provided simultaneously in the formation of CESL 52. It is observed that the low-frequency power has the effect of bombarding CESL 52, resulting in a greater density of the horizontal portions (top portions 52-1 and bottom portions 52-3) of CESL 52, while sidewall portions 52-2 are affected less by the bombardment, and hence have a lower density than that of top portions 52-1 and bottom portions 52-3. The low-frequency power may be increased relative to the high-frequency power to increase the densifying effect of top portions 52-1 and bottom portions 52-3. In the embodiment wherein both the high-frequency energy and the low-frequency energy are provided, a ratio of the high-frequency power to the low-frequency power may be lower than about 1, lower than about 0.8, or even lower than about 0.1.

Next, an isotropic etch is performed to remove sidewall portions 52-2 of CESL 52, while top portions 52-1 and bottom portions 52-3 are not removed. In an embodiment in which CESL 52 is formed of silicon nitride, the isotropic etch may be a wet etch using phosphoric acid. Since sidewall portions 52-2 have a lower density, they have a greater etching rate than that of top portions 52-1 and bottom portions 52-3. In the isotropic etch, top portions 52-1 and bottom portions 52-3 will also be reduced. However, the isotropic etch may be controlled so that at least some of top portion 52-1 and bottom portion 52-3 remain. FIG. 5A illustrates one embodiment wherein the remaining bottom portions 52-3 are spaced apart from spacers 26 and/or 46. FIG. 5B illustrates another embodiment, wherein remaining bottom portions 52-3 are in contact with spacers 26 and/or 46. The resulting top portions 52-1 may have a thickness greater than the thickness of bottom portions 52-3.

As a result of the removal of sidewall portions 52-2 of CESL 52, the aspect ratio (the ratio of height H to width W; refer to FIG. 5A) of gap 34 is reduced, and hence the possibility of forming voids in the subsequent gap-filling process is reduced. FIG. 6 illustrates the filling of inter-layer dielectric (ILD) 60, which is also referred to as ILD0 since an additional ILD will be formed thereon. ILD 60 may be formed of commonly used CESL materials including, but not limited to, SiN_(x), SiO_(x), SiON, SiC, SiBN, SiCBN, and combinations thereof. In an embodiment, ILD 60 is formed using HDP, although other methods such as SACVD, LPCVD, ALD, PEALD, PECVD, MLD, PICVD, spin-on, and the like may also be used.

Referring to FIG. 7, a chemical mechanical polish (CMP) may be performed to remove hard mask layers 24 and 44 and top portions 52-1. In alternative embodiments, the CMP may be performed using hard mask layers 24 and 44 as CMP stop layers. Next, gate dielectrics 20 and 40 and gate strips 22 and 42 are replaced by gate dielectrics 60 and 70 and metal gates 62 and 72. The formation processes are known in the art, and hence are not repeated herein. As a result, the gate stacks shown in FIG. 3B will also be replaced by gate dielectrics 60 and 70 and metal gates 62 and 72.

In subsequent process steps, as shown in FIG. 8, an additional ILD 74, also known as ILD1, is formed over ILD 60. The process is then continued by forming contact openings in ILDs 74 and 60 and filling the contact openings to form contact plugs 76. In the formation of the contact openings, bottom portions 52-3 of CESL 52 are used to stop the etching.

In alternative embodiments, as shown in FIG. 9, mask layers 24 and 44 (refer to FIG. 3A) are not formed, or are formed but removed before the formation of silicide regions. Gate silicides 68 and 78 may be formed on top of gate strips 22 and 42, respectively. In these embodiments, gate dielectrics 20 and 40 and gate strips 22 and 42 may not be replaced by gate dielectrics 60 and 70 and metal gates 62 and 72. Accordingly, top portions 52-1 of CESL 52 are used to stop etching in the formation of contact plugs 76 that are connected to gate silicides 68 and 78.

In accordance with one aspect of the embodiment, a method of forming an integrated circuit structure includes providing the integrated circuit structure having a first gate strip and a gate spacer on a sidewall of the first gate strip. A contact etch stop layer (CESL) is formed. The CESL includes a top portion directly over the first gate strip and a bottom portion lower than the top portion. The top portion and the bottom portion are spaced apart from each other by a space. A portion of a sidewall of the gate spacer facing the space has no CESL formed thereon.

In accordance with another aspect of the embodiment, an integrated circuit structure is provided. The integrated circuit structure includes a first gate strip; a gate spacer on a sidewall of the first gate strip; and a contact etch stop layer (CESL) having a bottom portion lower than a top surface of the gate spacer, wherein a portion of a sidewall of the gate spacer has no CESL formed thereon.

The embodiments of the present invention have several advantageous features. By removing sidewall portions of CESL 52, the aspect ratios of the gaps between adjoining gate spacers are reduced. Therefore, the gap filling is less likely to incur voids. This is particularly beneficial for MOS devices formed using the gate-last approach due to the relatively great height of the gate stacks.

According to an embodiment, an integrated circuit structure comprises a first conductive strip, a first spacer on a sidewall of the first conductive strip, a second conductive strip and a second spacer on a sidewall of the second conductive strip. A gap is between the first spacer and the second spacer. The structure further has a contact etch stop layer (CESL) comprising a top portion directly over the first conductive strip and a bottom portion in the gap and disconnected from the top portion, wherein a sidewall of the first spacer does not have any portion of the CESL formed thereon.

An integrated circuit structure according to an embodiment comprises a first gate strip on a substrate, a first spacer on a sidewall of the first gate strip, a second gate strip on the substrate and a second spacer on a sidewall of the second gate strip. A gap is between the first spacer and the second spacer. A contact etch stop layer (CESL) is disposed on the substrate between the first spacer and the second spacer and comprises a bottom portion lower than a top surface of the first spacer. A top surface of the bottom portion of the CESL is higher than a bottom surface of the first spacer and a bottom surface of the second spacer. A portion of a sidewall of the first spacer has no CESL formed thereon.

Other embodiments are also disclosed.

The advantageous features of the embodiments include a reduced aspect ratio of the gap between gate strips. As a result, it is easier to fill the gaps between the gate strips without causing voids.

Although the present invention and its advantages have been described in detail, it should be understood that various changes, substitutions and alterations can be made herein without departing from the spirit and scope of the invention as defined by the appended claims. Moreover, the scope of the present application is not intended to be limited to the particular embodiments of the process, machine, manufacture, and composition of matter, means, methods and steps described in the specification. As one of ordinary skill in the art will readily appreciate from the disclosure of the present invention, processes, machines, manufacture, compositions of matter, means, methods, or steps, presently existing or later to be developed, that perform substantially the same function or achieve substantially the same result as the corresponding embodiments described herein may be utilized according to the present invention. Accordingly, the appended claims are intended to include within their scope such processes, machines, manufacture, compositions of matter, means, methods, or steps. In addition, each claim constitutes a separate embodiment, and the combination of various claims and embodiments are within the scope of the invention. 

What is claimed is:
 1. A method comprising: providing a first gate strip over a substrate and having a gate spacer on a sidewall of the first gate strip; forming a contact etch stop layer (CESL) over the gate strip and on a first sidewall of the gate spacer, a second sidewall of the gate spacer is opposite the first sidewall of the gate spacer and nearest the first gate strip, wherein a top portion of the CESL extends over the gate strip and a bottom portion of the CESL extends over the substrate, and wherein a sidewall portion of the CESL is disposed on the first sidewall of the gate spacer; etching the CESL and removing at least the sidewall portion of the CESL on the first sidewall of the gate spacer; and forming an inter-layer dielectric (ILD) over and contacting the CESL; wherein the CESL comprises a material having a high etch selectivity with respect to the ILD; wherein, after the etching, the bottom portion of the CESL is lower than a top surface of the gate spacer; wherein, after the etching, a top surface of the bottom portion of the CESL is higher than a bottom surface of the gate spacer; and wherein, after the etching, at least a portion of the first sidewall of the gate spacer is exposed through the CESL.
 2. The method of claim 1, wherein the forming the CESL comprises using a deposition process with a low frequency energy to form the CESL, wherein the low frequency energy has a frequency lower than about 900 kHz.
 3. The method of claim 1, wherein the forming the CESL comprises using a deposition process with a high frequency energy and a low frequency energy to form the CESL, wherein the high frequency energy has a frequency higher than about 900 kHz and the low frequency energy has a frequency of about 350 kHz or lower.
 4. The method of claim 3, wherein a ratio of a power of the high frequency energy to a power of the low frequency energy is lower than about
 1. 5. The method of claim 1, further comprising providing a second gate strip adjacent the first gate strip with a gap between the first gate strip and the second gate strip, wherein the bottom portion of the CESL is in the gap.
 6. The method of claim 1, wherein after the etching the CESL, the bottom portion of the CESL adjoins the gate spacer.
 7. The method of claim 1, wherein after the etching the CESL, the bottom portion of the CESL is spaced apart from the gate spacer.
 8. The method of claim 1, wherein, after the etching, the top portion of the CESL is aligned directly over the first gate strip and disconnected from the bottom portion of the CESL.
 9. A method, comprising: providing a first conductive strip and a second conductive strip on a substrate; forming a first spacer on a first sidewall of the first conductive strip; forming a second spacer on a second sidewall of the second conductive strip, the first sidewall facing the second sidewall and the first spacer separated from the second spacer by a gap; forming a contact etch stop layer (CESL) over the first conductive strip and extending over the substrate in the gap, the CESL comprising a top portion directly over the first conductive strip, a bottom portion in the gap, and a sidewall portion disposed between the top portion and the bottom portion; modifying the CESL by removing the sidewall portion of the CESL such that the top portion and the bottom portion are separated, wherein the modifying the CESL comprises etching the CESL with an isotropic etch; and forming an inter-layer dielectric (ILD) over and contacting at least the bottom portion of the CESL.
 10. The method of claim 9, wherein the forming the CESL comprises using a deposition process with a low frequency energy source to form the CESL, wherein the low frequency source has a frequency lower than about 900 kHz, and wherein, after the forming the CESL, the sidewall portion of the CESL has a lower density than a density of the top portion and the bottom portion of the CESL.
 11. The method of claim 9, wherein the forming the CESL comprises using a deposition process with a high frequency energy and a low frequency energy to form the CESL, wherein the low frequency energy has a frequency of about 350 kHz or lower.
 12. The method of claim 11, wherein a ratio of a power of the high frequency energy to a power of the low frequency energy is lower than about
 1. 13. The method of claim 9, wherein, after the modifying the CESL, the bottom portion of the CESL is in contact with the first spacer and the second spacer.
 14. The method of claim 9, wherein modifying the CESL comprises removing a portion of the CESL such that the bottom portion of the CESL is separated and spaced apart from the first spacer and the second spacer.
 15. The method of claim 14, wherein forming the ILD comprises the ILD in the gap and extending between the first spacer and the bottom portion of the CESL, the CESL contacting the substrate between the first spacer and the bottom portion of the CESL.
 16. The method of claim 9, wherein the sidewall portion of the CESL is disposed directly on the first spacer, wherein, after the modifying the CESL, the first spacer is exposed through the CESL.
 17. A method, comprising: forming a first gate strip on a substrate; forming a gate spacer on a sidewall of the first gate strip, the gate spacer having a first sidewall opposite a second sidewall, the second sidewall of the gate spacer nearest the first gate strip; forming a contact etch stop layer (CESL) over the gate strip and extending over the first sidewall of the gate spacer and over the substrate adjacent to the gate strip; removing a sidewall CESL portion to expose at least a portion of the first sidewall of the gate spacer, the removing forming a top CESL portion separated from a bottom CESL portion; and forming an inter-layer dielectric (ILD) over and contacting the CESL; wherein, after removing the sidewall portion of the CESL, a top surface of the bottom CESL portion is higher than a bottom surface of the gate spacer.
 18. The method of claim 17, wherein the forming the CESL comprises using a deposition process with a low frequency energy to form the CESL having the sidewall CESL portion with a lower density than the top CESL portion and the bottom CESL portion.
 19. The method of claim 18, wherein the low frequency energy has a frequency of about 350 kHz or lower.
 20. The method of claim 17, wherein the forming the CESL comprises using a deposition process with a high frequency energy and a low frequency energy to form the CESL, wherein the low frequency energy has a frequency of about 900 kHz or lower, and wherein a ratio of a power of the high frequency energy to a power of the low frequency energy is lower than about 0.8. 